The present invention relates to a nonvolatile semiconductor memory, and also relates to a method of writing data to the nonvolatile semiconductor memory.
Various types of nonvolatile semiconductor memory are known in the art. One type of nonvolatile semiconductor memory includes a plurality of memory cells each composed of a single MOS (metal oxide semiconductor). The memory cell has a drain region and a source region, and these regions have respective charge accumulating parts for accumulating electric charges, thereby enabling storage of bit data of binary values (0, 1), i.e., storage of two-bit data. Such nonvolatile semiconductor memory is disclosed in “Four-bit per Cell NROM Reliability,” by Eitan, Boaz and other eleven authors, IEEE International Electron Devices Meeting 2005, iedm Technical Digest, Washington, D.C., Dec. 5-7, 2005 in the United States, IEEE 2005 Session 22.1. Also, such nonvolatile semiconductor memory is disclosed in PCT/US2005/046653 or WO 2006/076145.
In the above-described nonvolatile semiconductor memory, a state where charges (electrons) are not accumulated in the charge accumulating parts is defined as an initial state. The state where no charges are accumulated corresponds to data “1,” and a state where charges are accumulated corresponds to data “0.”
Data can be written into and read out of the nonvolatile semiconductor memory, and data in the nonvolatile semiconductor memory is erased in the following manner.
Data “0” is written into the charge accumulating part of the drain by applying a positive voltage to the drain region, applying a positive voltage to a gate electrode, and applying a ground voltage to the source region. Upon application of these voltages, hot electrons are injected into the charge accumulating part of the drain to write data “0.” Data is read from the drain by applying a positive voltage to the source region, applying a positive voltage to the gate electrode, and applying a ground voltage to the drain region. If no charges are accumulated in the charge accumulating part of the drain, a current higher than a threshold is read out. In this case, it is determined that data “1” is read out. If charges are accumulated in the charge accumulating part of the drain, the value of a current to be read out is lower than the threshold. In this case, it is determined that data “0” is read out. Accordingly, determination of whether binary data is “0” or “1” is made by determining whether or not a read-out current value is less than a predetermined threshold.